Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued 15 Aug. 1989, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate, on which are arranged a matrix of conductors. The first substrate is also called the cathode plate or the emitter plate. In one direction of the matrix, conductive columns comprising the cathode electrode support the microtips. In the other direction, above the column conductors, are perforated conductive rows comprising the grid electrode. The row and column conductors are separated by an insulating iayer having apertures permitting the passage of the microtips, each intersection of a row and column corresponding to a pixel.
On a second substrate, facing the first, the display has regularly spaced, parallel conductive stripes comprising the anode electrode. The second substrate is also called the anode plate. These stripes are alternately covered by a first material luminescing in the red, a second material luminescing in the green, and a third material luminescing in the blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
The Clerc patent discloses a process for addressing a trichromatic field emission flat panel display. The process consists of successively raising each set of interconnected anode stripes periodically to a first potential which is sufficient to attract the electrons emitted by the microtips of the cathode conductors corresponding to the pixels which are to be illuminated in the color of the selected anode stripes. Those anode stripes which are not being selected are set to a potential such that the electrons emitted by the microtips are repelled or have an energy level below the threshold cathodoluminescence energy level of the luminescent materials covering those unselected anodes.
Luminescence is a characteristic nonthermal emission of electromagnetic radiation by a material upon some form of excitation. Thus, luminescence is the conversion. of energy into light without heat. The luminescence type is usually defined by the excitation means. For example, cathodoluminance is where the source of energy is cathode rays. The luminescence process itself involves (1) the absorption of energy; (2) excitation; and (3) the emission of energy, usually in the form of radiation in the visible portion of the spectrum; however, the emission can also be in the infrared or ultraviolet portions of the spectrum. Visible light constitutes one part of the electro-magnetic spectrum (approximately 4,000 .ANG.-8,000 .ANG.).
When the luminance persists after the excitation is removed it is called phosphorescence. Quantitatively, phosphorescence may be defined as luminescence that is delayed by more than 10.sup.-8 seconds after excitation. An inorganic luminescent material, such as phosphor, usually consists of a crystalline host lattice to which is added a trace of impurities, called the activator and co-activator. The activator is usually present in concentration levels varying from a few parts per million to one or two percent of the host lattice. Co-activators are the additional impurities which act as charge compensators or donors in the lattice.
Referring initially to FIG. 1, there is shown, in cross-sectional view, a portion of an illustrative prior field emission device in which the present invention may be incorporated. This device comprises an anode plate 1 having a cathodoluminescent phosphor coating 3 facing an emitter plate 2, the phosphor coating 3 being observed from the side opposite to its excitation.
More specifically, the field emission device of FIG. 1 comprises an anode plate 1 and an electron emitter (or cathode) plate 2. A cathode portion of emitter plate 2 includes conductors 9 formed on an insulating substrate 10, an electrically resistive layer 8 which is formed on substrate 10 and overlaying the conductors 9, and a multiplicity of electrically conductive microtips 5 formed on the resistive layer 8. In this example, the conductors 9 comprise a mesh structure, and microtip emitters 5 are configured as a matrix within the mesh spacings. Microtips 5 take the shape of cones which are formed within apertures through conductive layer 6 and insulating layer 7.
A gate electrode comprises the layer of the electrically conductive material 6 which is deposited on the insulating layer 7. The thicknesses of gate electrode layer 6 and insulating layer 7 are chosen in such a way that the apex of each microtip 5 is substantially level with the electrically conductive gate electrode layer 6. Conductive layer 6 may be in the form of a continuous layer across the surface of substrate 10; alternatively, it may comprise conductive bands across the surface of substrate 10.
Anode plate 1 comprises a transparent, electrically conductive film 12 deposited on a transparent planar support 13, such as glass, which is positioned facing gate electrode 6 and parallel thereto, the conductive film 12 being deposited on the surface of the glass support 13 directly facing gate electrode 6. Conductive film 12 may be in the for. m of a continuous layer across the surface of the glass support 13; alternatively, it may be in the form of electrically isolated stripes comprising three series of parallel conductive bands across the surface of the glass support 13, as shown in FIG. 1 and as taught in U.S. Pat. No. 5,225,820, to Clerc. By way of example, a suitable material for use as conductive film 12 may be indium-tin-oxide (ITO), which is substantially optically transparent and electrically conductive. Anode plate 1 also comprises a cathodoluminescent phosphor coating 3, deposited over conductive film 12 so as to be directly facing and immediately adjacent gate electrode 6. In the Clerc patent, the conductive bands of each series are covered with a particulate phosphor coating which luminesces in one of the three primary colors, red, blue and green 3.sub.R, 3.sub.B, 3.sub.G.
Selected groupings of microtip emitters 5 of the above-described structure are energized by applying a negative potential to cathode electrode 9 relative to the gate electrode 6, via voltage supply 15, thereby inducing an electric field which draws electrons from the apexes of microtips 5. The potential between cathode electrode 9 and gate electrode 6 is approximately 70-100 volts. The emitted electrons are accelerated toward the anode plate 1 which is positively biased by the application of a substantially larger positive voltage from voltage supply 11 coupled between the cathode electrode 9 and conductive film 12 functioning as the anode electrode. The potential between cathode electrode 9 and anode electrode 12 is approximately 300-1000 volts. Energy from the electrons attracted to the anode conductive film 12 is transferred to particles of the phosphor coating 3, resulting in luminescence. The electron charge is transferred from phosphor coating 3 to conductive film 12, completing the electrical circuit to voltage supply 11. Charge can also be transferred by secondary electron emission. The image created by the phosphor stripes is observed from the anode side which is opposite to the phosphor excitation, as indicated in FIG. 1.
The process of producing each frame of a display using a typical trichromatic field emission display includes (1) applying an accelerating potential to the red anode stripes while sequentially addressing the gate electrodes (row lines) with the corresponding red video data for that frame applied to the cathode electrodes (column lines); (2) switching the accelerating potential to the green anode stripes while sequentially addressing the rows lines for a second time with the corresponding green video data for that frame applied to the column lines; and (3) switching the accelerating potential to the blue anode stripes while sequentially addressing the row lines for a third time with the corresponding blue video data for that frame applied to the column lines. This process is repeated for each display frame.
It is to be noted and understood that true scaling information is not intended to be conveyed by the relative sizes and positioning of the elements of anode plate 1 and the elements of emitter plate 2 as depicted in FIG. 1. For example, in a typical FED shown in FIG. 1 there are approximately one hundred arrays 4, of microtips per display pixel, and there are three color stripes 3.sub.R, 3.sub.B, 3.sub.G per display pixel. Furthermore, phosphor coating 3 may not be a dense coating, but instead be comprised of an arrangement of phosphor particles which have adhered to conductors 12.
The phosphor coating 3 of the anode plate 1 could be applied to the conductors 12 using the well known method of electrophoretic deposition. In general the process involves suspending phosphor powders in a nonaqueous polar organic liquid containing small quantities of dissociating salts. The cations from the added salts absorb onto the phosphor particles creating a net positive charge. These positively charged particles deposit on the anode stripes 12, which is the cathode during electrophoresis, when a potential is applied. In summary, the electrophoretic deposition process includes the charging of the particles to be deposited, the transport of the particles to the substrate, and the collection and adherence of the particles on the substrate.
Little is known regarding the true mechanism of phosphor adherence to the anode stripe conductors. Some scientists believe that the ions in the phosphor bath not only act to charge the phosphor positively, but also to form an adhesive oxide matrix which binds the phosphor particles to the conductors. Others theorize that the phosphor particles adhere to the conductor surface by electrostatic attraction.
The typical phosphor synthesis process creates a non-active surface layer (often called a "dead voltage layer") on the phosphor particles. This inactive surface layer, also referred to as `skin` herein, contributes significantly to phosphor inefficiency. It is well known that the penetration depth of an electron into a phosphor particle is related to the kinetic energy of the electron. The penetration depth (called the mean-free path) of the electrons in the typical FED application is approximately 100 .ANG.. Since thickness of the phosphor coating can be greater than 100 .ANG., a significant number of the electrons will recombine nonradiatively in the surface layer and will not produce luminance from the phosphor. The occurrence of nonradiative recombinations contributes to phosphor inefficiency and therefore adversely impacts display brightness and quality. The inactive surface region of the phosphor has a greater adverse effect in FED applications than in Cathode Ray Tube (CRT) applications because CRT's typically operate at a much higher voltage (25-30 kilovolts) and therefore the mean-free path is much greater and the phosphor efficiency is higher in CRT applications.
Luminous efficiency is defined as the ratio of the total luminous flux in lumens emitted by a light source over all wavelengths to the total incident energy in watts (current.times.volts). The value for lumens is adjusted to take into account the efficiency of the human eye.
It is well known that most of the commonly used flat panel display phosphors have a lower luminance efficiency at the acceleration voltage levels of the typical field emission device (below 1 kV) compared to other systems such as the Cathode Ray Tube (CRT) (25-30 kV). It is advantageous to operate the field emission device at the lower voltages because the low voltage operation simplifies spacer technology, reduces driver and interconnect cost, reduces display mortality caused by high voltage arcing, and allows the use of the switched anode design. Therefore, one shortcoming of field emission displays of the current technology is the reduced phosphor efficiency caused by the relatively low accelerating voltage between the cathode and anode plates. An improved luminance efficiency would facilitate improved display luminance and reduced power consumption.
In view of the above, it is clear that there exists a need for improved phosphor efficiency for field emission devices. More specifically, what is needed is an improvement in the structure and method of manufacturing the anode plate of a field emission flat panel display device which facilitates improved phosphor efficiency and higher luminance.